Switch mode power supplies or switching regulators, also referred to as DC to DC converters, are used to convert an input supply voltage to a desired output voltage at a voltage level appropriate for integrated circuits in an electronic system. For example, a 12 volts supply voltage provided to an electronic system may need to be reduced to 5 volts for supplying the I/O interface circuits and reduced to 1V for supplying the core digital logic circuits. A switching regulator provides power supply function through low loss components such as capacitors, inductors, and transformers, and power switches that are turned on and off to transfer energy from the input to the output in discrete packets. A feedback control circuit is used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.
A switching regulator can be configured to step up the input voltage or step down the input voltage or both. Specifically, a buck switching regulator, also called a “buck converter,” steps down the input voltage while a boost switching regulator, also called a “boost converter,” steps up the input voltage. A buck-boost switching regulator, or buck-boost converter, provides both step-up and step-down functions.
The operation of the conventional switching regulator is well known and is generalized as follows. A power switch is turned on periodically to apply energy to an inductor of an output filter circuit to allow the current through the inductor to build up. When the power switch is turned off, the voltage across the inductor reverses and charges are transferred onto an output capacitor of the output filter circuit and the load. A relatively constant output voltage is maintained by the output capacitor. A second power switch, sometimes called a synchronous rectifier, is sometimes used for synchronous control operation. In general, the main power switch, also referred to as the high-side switch, is turned on while the second power switch, referred to as the low-side switch, is turned off, and vice versa.
Switching regulators include a control circuit which typically uses an error amplifier to compare a feedback voltage indicative of the output voltage with a reference voltage and the control circuit generates one or more control signals that control the switching frequency (pulse frequency modulation) or the pulse width (pulse width modulation) of the on-off switching cycle. Many different control schemes have been applied to control the duty cycle (i.e., the on-time) of the main power switch. A constant on-time (or fixed on-time) control scheme is one type of control schemes where the on-time of the main power switch of the switching regulator is kept constant and the off time of the main power switch is varied to generate the desired output voltage. Constant on-time control scheme is preferred in the industry for some important advantages, such as good light load efficiency and faster transient response.
FIG. 1 is a schematic diagram of a conventional switching regulator implementing a constant on-time control scheme. In switching regulator 10, a main power switch M1 and a second power switch M2 are connected in series between the input voltage VIN (node 12) and the ground potential. Power switch M1 operates to switch the input voltage VIN to an inductor L1 periodically to charge inductor L1. When the main power switch is turned off, the energy stored in the inductor L1 is transferred to an output capacitor COUT and a load 18 and a substantially constant output voltage VOUT is maintained. The inductor L1 and the output capacitor COUT form a low-pass filter. The second power switch M2 is used to realize synchronous rectification and is driven by the inverse of the drive signal (node 32) driving the main power switch M1. Thus, the low-side power switch M2 is turned on when the main power switch M1 is turned off and vice versa.
A regulator control circuit is configured to drive the power switches M1 and M2 according to a constant on-time control scheme. In operation, the main power switch M1 is turned on for a fixed time duration as determined by a one-shot timer 26 and switch M1 is then turned off. The output voltage VOUT (node 16) is monitored through a feedback loop. More specifically, the output voltage VOUT is fed back to the regulator control circuit as a feedback voltage VFB. The feedback voltage VFB is compared with a reference voltage VREF at a voltage comparator 22. The output of the voltage comparator 22 is gated with the output of a minimum off-time timer 30 at an AND logic gate 24. After the main power switch M1 has been turned off for at least the minimum off-time, when the output voltage VOUT decreases below the reference voltage VREF, AND gate 24 will trigger the one-shot timer 26 to turn on the main power switch M1 again for the fixed on-time duration. The conventional switching regulator 10 realizes fast transient response and high efficiency at light load condition.
Fixed on-time (or constant on-time) regulators are one type of voltage regulators employing ripple-mode control. In general, ripple-mode regulators regulate their output voltage based on the ripple component in the output signal. Because of the switching action at the power switches, all switch-mode regulators generate an output ripple current through the switched output inductor. This current ripple manifests itself as an output voltage ripple due, principally, to the equivalent series resistance (ESR) in the output capacitor placed in parallel with the load.
For voltage regulators using ripple-mode control, while the output ripple is useful in output voltage regulation, it is undesirable in terms of output signal noise and load voltage limits. Indeed, the desire to minimize output ripple has lead to design and production of capacitors having very low ESR. Lowering output capacitor ESR can significantly lower the output ripple signal. Low ripple serves the interests of noise minimization and reduced load voltage variation, but makes ripple-mode regulation more difficult. Low ripple magnitude reduces the comparator voltage differentials, making accurate and fast comparison very difficult.
To that end, manufacturers of fixed on-time voltage regulators often impose a minimum ESR for the output capacitor to ensure a minimum amount of ripple voltage at the output voltage so that effective ripple-mode control can be realized. Thus, an output capacitor with a large ESR has to be used with all fixed on-time voltage regulators. In some cases, when the output capacitor itself does not have enough ESR, manufacturers suggest including a resistor in series with the output capacitor to introduce enough series resistance to generate the required minimum amount of ripple voltage.
The requirement of a minimum amount of ripple voltage at the output signals limits the application of fixed on-time voltage regulators to cases where ripples in the output voltage can be tolerated. Also, zero ESR capacitors, such as ceramic capacitors, which are usually cheaper than tantalum capacitors having large ESR, cannot be used because a minimum amount of ESR is required for proper control loop operation.
Solutions to enable a fixed on-time voltage regulator to use a low ESR output capacitor have been proposed. For example, in some cases, a virtual ripple generator is used to generate an internal virtual ripple proportional with the inductor current. While these solutions allow for the use of low ESR capacitor in ripple-mode voltage regulators, these solutions add complexity and cost to the voltage regulators.
In another example, a buck voltage regulator varies the reference voltage within a pulse width modulation (PWM) cycle to generate the PWM signal. FIG. 2, which includes FIGS. 2(a) and 2(b), illustrates the signal waveforms for a buck voltage regulator using a voltage ramp in one example. In particular, the reference voltage is maintained at a low fixed value when the main power switch is conducting and then the reference voltage is ramped up during the off-time of the main power switch to a final value, as shown in FIG. 2(a). While this type of voltage ramp is capable of providing compensation to the feedback control loop of the voltage regulator, the voltage ramp for different output voltages will have different final voltage values so that a voltage offset results for different output voltages, as shown in FIG. 2(b). For different output voltages, the voltage regulator will be operated at different duty cycle and the off-time of the main switch varies. As a result, the voltage ramp will have different final voltage values for different output voltages. The voltage offset between different output voltages causes manufacturers to require the user to modify the fixed reference voltage value based on the output voltage selected. This type of ad-hoc user modification is inconvenient and a burden to the user.